In the related art, a printer, a copy machine, and a complex machine (multi-functional peripheral (MFP)) which use an electrophotographic process are known. As exposing means (exposing unit) of these equipments, there are two types called a laser optical system (laser scan unit (LSU)) and a print head (solid head). In the laser optical system, a photoconductive drum is exposed by being scanned with a laser light beam using a polygon mirror. In the print head, the photoconductive drum is exposed by light beams output from a plurality of light emitting elements such as a light emitting diode (LED).
In the laser optical system, the polygon mirror is required to be rotated at high speed, and thus much energy is consumed and operation sound is generated when an image is formed. Also, since a mechanism for scanning with laser light is required, a unit tends to be large.
Meanwhile, the print head has a structure in which light beams emitted from the plurality of light emitting elements form an image on the photoconductive drum using a small size lens forming an erected image, which is called a rod lens array. Because of this structure, the print head can be miniaturized. In addition, there is no operating unit, and thus the exposing unit makes less noise.
As the print head, a print head using an organic EL (organic light emitting diode (OLED)) is also developed in addition to a print head using an LED (LED chips are arranged). As the print head using the LED, a print head in which LED chips are arranged on a print substrate is generally used. In the organic EL (OLED), the organic ELs are collectively formed on the substrate using a mask, and the light emitting elements can be accurately arranged therein. When the organic EL is used as the light emitting element, there is merit in that an image is formed with high accuracy. For example, an example is known in which the plurality of light emitting elements constituted by the organic EL are formed on a glass substrate.
In order to form a good image by the print head using the LED or the organic EL (OLED), after light beams output from the plurality of light emitting elements pass through the rod lens array, light intensity (amount of light) or a diameter of the light beam is required to be uniform on the photoconductive drum.
In order to uniform the light intensity (amount of light) or the diameter of the light beam on the photoconductive drum, light intensity (amount of light) of each light emitting element is generally controlled.
In order to control the light intensity (amount of light) of each light emitting element, a method such as a control of an amount of the current flowing in each light emitting element or a control of light emitting time per unit time is adopted.
For example, the light intensity (amount of light) or the diameter of the light beam on the photoconductive drum is uniformed by individually setting the amount of the current flowing in each light emitting element or light emitting time per unit time in every light emitting element.
For example, an example is known in which the amount of light (light intensity) or a diameter of a dot (diameter of light beam) is controlled by controlling the amount of the current flowing in the light emitting element.
In addition, for example, it is known that, the greater the current flowing in the light emitting element, the faster degradation of the amount of light of the light emitting element (organic EL element).